A swivel is a rotatable pipe joint typically used on production ships extracting oil and gas from submerged wells. It is mounted on a turret transferring the oil and gas from one or more risers to pipelines being connected to storage tanks, processing equipment or the like on the ship. Stacks of swivels can be assembled within a turret structure. Conventional lifting equipment can mount and demount them.
Small scale swivels are conventionally used in existing production ship installations. Many ships using these swivels are located in less turbulent waters or working in fields with fewer demands on ship availability. Because of their size, the swivels are easily handled at sea by conventional equipment, provided there is good weather.
Small scale swivels cannot, however, efficiently handle production from planned large capacity installations. Prospective swivels, with a characteristic weight of 50 to 100 tons, and correspondingly cumbersome dimensions, are being designed for these installations. Conventional equipment will not be able to quickly and efficiently replace heavy swivels at sea.
Many operators replace swivels at a shipyard, often during overhauling. The assumption is that swivel replacement is very dependent on external conditions. But onshore replacement stops production, or at least eliminates ship availability. Because of these inherent delays, some operators are now requiring production ships to carry suitable lifting equipment to replace the swivels.
The best solution for heavy swivels is onboard disassembly and replacement, sending onshore only the parts that are not reparable onboard. Further, disassembly should be performed at a distance from the turret so that production is not impeded.